This invention relates to a piston ring fitted on a piston reciprocating in a cylinder and, more particularly, to a piston ring which is suitable for use in a piston type compressor such as a slant plate type compressor.
In a compressor such as a slant plate type compressor, lubrication for the driving mechanism in the crank chamber is generally supplied by blow-by gas which is mixed with lubricating oil in a mist state. The crank chamber communicates with the suction chamber through a passageway. The blow-by gas returns to the suction chamber through the passageway to maintain certain pressure in the crank chamber.
As mentioned above, the blow-by gas is very important for operating the compressor. Therefore, amount of blow-by gas leaked into the crank chamber should be maintained at a constant level. Accordingly, a conventional piston ring is fitted in an annular groove which is made on an outer surface of each of the pistons.
For example, a piston ring as the conventional piston ring is disclosed in U.S. Pat. No. 4,835,856 (corresponding to J-P-A 63-13639) issued to Katsumasa Azami et al and assigned to SANDEN CORPORATION. The piston ring is provided with a plurality of communicating grooves in its outer peripheral surface ot communicate between the interior of the crank chamber and a compression space of each of the piston cylinders to insure the constant amount of blow-by gas. As a result, the lubrication oil smoothly flows from the compression space into the crank chamber through the communicating grooves of the piston ring with the piston reciprocating in each piston cylinder.
When the piston is reciprocated in each piston cylinder to carry out a compression, the piston ring is subjected to external force due to a differential pressure between the compression space and the crank chamber. Therefore, the piston ring is repeatedly moved in the annular groove of the piston in various directions. This results in abrasion of both of the piston and the piston ring. In order to reduce the abrasion, it is desirable to supply the lubricating oil into the annular groove.